To identify potential selection pressures which lead to RNA sequence conservation, we examined the occurrence rates of dinucleotides in 64 single-stranded RNA virus genomes. These viruses may offer a particular insight into these pressures since their RNA-dependent RNA polymerases lack proofreading capability. This potentiates introduction of mutations into their genomes, yet unidentified selection processes conserve the genomes to a large degree. We report a strong inverse correlation between the C+G content and the occurrence of the CpG dinucleotide (r=0.71) in the RNA virus genomes, in contrast to earlier reports (Karlin et al., 1994, Journal of Virology 68, 2889-2897). We also detected significant suppression of UpA, correlating inversely with genomic U+A content. These suppressions are coupled with over-representation of the complementary pair of dinucleotides, CpA and UpG. In addition, we highlight the fact that odds ratios for dinucleotides are not independent variables, a situation apparently not widely appreciated in the literature. This led us to view the over-representation of CpA and UpG as a consequential outcome of UpA and CpG suppression in the virus genomes. Potential factors influencing these disturbances are discussed. In addition, higher than random incidence was observed for 'out-of-frame' stop codons in the viral RNA genomes, with some preferences for individual codons being exhibited by certain virus groups. The UAG codon appeared more common in the +1 frame, the UGA in the -1 frame.


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